1. Field of the Invention
The present invention relates to a profile measuring instrument. More specifically, the invention relates to an instrument for measuring a surface profile of an object with a high accuracy.
2. Description of Related Art
A typical instrument for highly accurately measuring a surface profile of an object (workpiece) includes variety of instruments depending on the degree of irregularities of the workpiece surface to be measured. For instance, a surface roughness measuring instrument has been used for quantitatively measuring a roughness of a workpiece surface, whereas a scanning probe microscope has been used for observing an atomic-level irregularities of a workpiece.
These highly accurate profile measuring instruments scan a surface of a workpiece in a predetermined scan direction with a probe and detect a profile of the workpiece surface as a function of the position in the scan direction and a displacement of the probe on the workpiece surface.
During the scanning, the workpiece surface is optically scanned or a minute stylus (probe) supported by a cantilever is used for tracing the workpiece surface while optically detecting a displacement (corresponding to a displacement of a stylus tip tracing the workpiece surface) of a predetermined portion located on a backside (i.e. a side opposite to a side facing the stylus and the workpiece) of the cantilever.
Various improvements have been made in the profile measuring instruments in response to a demand for enhancing the accuracy thereof (see Literature 1: JP-A-2008-51602). In the Literature 1, the inventors of the present application have noted a motion error during the above-described scanning of the workpiece surface and proposed a profile measuring instrument having a unique structure for eliminating the influence of the motion error.
Specifically, the above-described scanning profile measuring instrument requires a scanning mechanism for moving a displacement sensor in order to scan the workpiece surface, where the motion error in the scanning mechanism is inevitable. Such a motion error is detected by the displacement sensor as an additional component to the displacement of the stylus in accordance with irregularities on the workpiece surface. In other words, the displacement detected by the displacement sensor is influenced by the motion error as compared to the true displacement of the stylus in accordance with the irregularities on the workpiece surface.
In contrast, when the displacement sensor is a device that detects a displacement of a measurement target portion relative to a reference member (e.g. a laser interferometer), the influence of the motion error in the data detected by the displacement sensor can be canceled by holding the reference member relative to the workpiece in a manner that the position and attitude of the reference member stay constant.
In the invention disclosed in the Literature 1, the reference member is supported so that the position and attitude of the reference member stay constant relative to the workpiece as discussed above, thereby eliminating the motion error of the scanning mechanism.
Specifically, a reference mirror (reference member) is supported along a workpiece surface, a stylus is supported via a cantilever between the reference mirror and the workpiece, and a laser interferometer is disposed opposite to the workpiece relative to the reference mirror. The laser beam from the laser interferometer is reflected by the reference mirror to provide the reference beam. A part of the laser beam is transmitted through the reference mirror to be reflected by a predetermined portion on a backside of the cantilever to provide measurement beam. The stylus and the laser interferometer are moved along the workpiece surface and the reference mirror for scanning by a scanning mechanism. The laser interferometer compares the measurement beam and the reference beam to measure the displacement of the stylus on the workpiece surface.
According to the above arrangement, even when a motion error is caused in the scanning mechanism while the workpiece surface is scanned, the influence acts on both of the measurement beam and the reference beam. Thus, the influence is cancelled when comparing the measurement beam and the reference beam and does not appear on a profile measurement data of the workpiece surface obtained by the laser interferometer.
It should be noted that the displacement sensor exemplified in the Literature 1 is an optical interferotype displacement meter, which is specifically a Fizeau laser interferometer that uses a laser beam and the reference mirror is disposed in an optical path of the measurement beam.
However, since the backside of the stylus (measurement target portion) is disposed in an extension of the optical path of the reference beam (i.e. reference optical path, from a light source to the reference mirror) and the optical path of the measurement beam (i.e. measurement optical path, from the light source to the measurement target portion) is partially shared by the reference optical path in the Fizeau laser interferometer, the length of the measurement optical path inevitably becomes longer than the reference optical path by the distance between the reference mirror and the measurement target portion. The optical path length difference between the measurement optical path and the reference optical path is referred to as a dead path.
Under the presence of the dead path, stability of a light source influences on a measurement error.
In other words, even without irregularities on the object to be measured, the fluctuation in the frequency of the laser creates an apparent difference in the optical path length, which appears as a measurement error.
Specifically, during length measurement with a laser interferometer, the apparent optical path length varies depending on the frequency stability of a laser (light source) and the length of the above-described dead path.
For instance, when the frequency stability of the laser is 1×10−6 and supposing that the length of the dead path is 100 mm, an apparent variation of the optical path length is calculated as: 1×10−6×100×10−3 m=100×10−9 m=100 nm.
The apparent variation of the optical path length depending on the frequency stability of the laser beam is a cause of an error of 100 nm due to the frequency fluctuation of the laser beam even without irregularities on the object to be measured.
In order to reduce the measurement error due to the frequency stability of the laser beam, the length of the dead path may be reduced or a frequency-stabilized laser having high frequency stability may be used.
However, the above solutions respectively accompany the following problems.
With regard to the reduction in the length of the dead path, optical elements (e.g. a lens and wave plate) for concentrating the laser beam onto the backside of the cantilever have to be disposed between the reference mirror and the backside of the stylus in the Fizeau laser interferometer. Since a space for receiving the optical elements and holders thereof is required, it is difficult to reduce the length of the dead path (i.e. the distance between the reference mirror and the backside of the stylus) to an order of, for instance, several millimeters.
With regard to the frequency stability of the laser beam, a laser source of which frequency stability is in an order of 1×10−9 is currently commercially available, which can be used for stabilizing the laser beam frequency. However, all of these frequency-stable laser sources are as expensive as approximately JPY one million.
On the other hand, since the frequency stability of an inexpensive semiconductor laser source is approximately 1×10−3 and the frequency stability of a He—Ne laser of which frequency is not stabilized is approximately 1×10−6, in order to reduce the influence on the measurement error, the length of the dead path has to be significantly reduced. Thus, a Fizeau laser interferometer which requires the above-described space for disposing the optical elements cannot be constructed.
As described above, the arrangement disclosed in the Literature 1 accompanies the problem of the presence of the dead path, where stability of a light source influences on a measurement error. In addition, the device exemplified in the Literature 1 accompanies the following problem for supporting the reference mirror.
Specifically, in the invention disclosed in the Literature 1, in order to hold the reference mirror (reference member) so that the position and attitude of the reference mirror stay constant relative to a workpiece, the workpiece is mounted on a base and the reference mirror is supported on the base via a plurality of columns (holder member), whereby the workpiece is covered with the reference mirror.
According to the above structure, a measurable area on a workpiece surface is limited to an area surrounded by the plurality of columns supporting the reference mirror.